The present invention relates generally to a device for purifying fuel cell reformate, and more particularly to the integration of shift, membrane and methanation reactors into a compact housing to facilitate carbon monoxide removal.
While conventional power sources devices (such as internal combustion engines, including piston and gas turbine-based platforms) are well-known as ways to produce, among other things, motive, heat and electric power, recent concerns about the effects they and their fuel sources have on the environment has led to the development of alternative means of producing such power. The interest in fuel cells is in response to these and other concerns. One form of fuel cell, called the proton exchange membrane (PEM) fuel cell, has shown particular promise for vehicular and related mobile applications. A typical PEM construction includes an anode and a cathode, with a solid polymer electrolyte membrane spaced between them such that protons generated at the anode can travel through the electrolyte and to the cathode. In PEM fuel cells, hydrogen or a hydrogen-rich gas is supplied to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied to the cathode side of the fuel cell. Catalysts, typically in the form of a noble metal (such as platinum), are placed at the anode and cathode to facilitate the ionization of hydrogen and subsequent reaction between the hydrogen and oxygen. While much of the present disclosure is in the context of PEM fuel cells, it will be appreciated by those skilled in the art that the invention disclosed herein has utility in other forms of fuel cells, where clean-up of fuel precursors can be used for improved fuel cell system operability, as well as for other processes where highly purified hydrogen feedstock is necessary.
In an ideal fuel supply situation, pure hydrogen gas is used as a direct fuel source. This is impractical in many vehicle-based fuel cell systems, as the amount of gaseous hydrogen required to be carried in order to achieve adequate vehicle range between refueling stops would be prohibitively large. A promising alternative to the direct feeding of hydrogen is the reformation of on-board liquid hydrocarbons through a fuel processing system upstream of the fuel cell. Methanol is an example of a readily-available hydrocarbon fuel, and accordingly has become one of the preferred hydrogen precursors, especially for volume-constrained mobile fuel cell applications. Its relative low cost and liquid state at practical temperatures of interest make it compatible with existing fuel delivery infrastructure. Unfortunately, during the conversion of methanol to hydrogen, carbon monoxide is also produced, of which even minute amounts can poison the noble metal catalyst on the downstream fuel cell anode and cathode. Accordingly, it becomes necessary to reduce the concentration of carbon monoxide to an acceptable level.
A typical fuel processing system incorporating methanol as the feed stock includes a reformer and one or more purification stages. There have emerged three general types of reformers that can be used on methanol and related liquid hydrocarbons: (1) steam reforming; (2) partial oxidation reforming; and (3) autothermal reforming. In the first variant, a pre-heated mixture of fuel and steam is reacted, while in the second variant, a pre-heated mixture of fuel and air is reacted. The third variant combines elements of both processes in a single reactor, and using a specially designed catalyst, enables balancing of the endothermic first and exothermic second variants. In all three cases, a reformate containing the desired end product, gaseous hydrogen, as well as undesirable carbon monoxide, is produced. A shift reactor may be employed to convert the carbon monoxide in the reformate. It has been found that to promote the following reaction in the direction shown,CO+H2O→CO2+H2,the reformate should be cooled prior to sending it to the shift reactor. Serially connected shift reactors may be used to further reduce the carbon monoxide concentration. While this level of carbon monoxide cleanup could be sufficient for certain types of fuel cells, it is still not adequate for others, such as PEM fuel cells. Accordingly, additional steps must be taken to ensure that the concentration of carbon monoxide in the reformate is further reduced. Three common approaches exist for achieving the exceptionally low carbon monoxide concentrations necessary for proper PEM operation. In one method, carbon monoxide can be reacted with hydrogen, typically in the presence of a catalyst, to produce methane and water:CO+3H2→CH4+H2Oin what is termed a methanation reaction. In another method, thin hydrogen-permeable noble metal membranes deposited onto a porous carrier can be used to promote the diffusion and consequent purification of hydrogen in the reformate. The third method involves the selective oxidation of the carbon monoxide in the presence of a noble metal catalyst as follows:2CO+O2→CO2.
Furthermore, two or more of these approaches may be used sequentially to achieve the desired level of carbon monoxide reduction. For example, the methanation device can be placed downstream (i.e., at the permeate side) of the membrane to react with any carbon monoxide that manages to get past the membrane. While the aforementioned approaches are capable of achieving aggressive carbon monoxide reduction goals, their inclusion results in added weight, volume and complexity to the fuel cell system. Accordingly, there exists a need to reduce the concentration of carbon monoxide in the reformate to very low levels while simultaneously minimizing the weight, cost, complexity and space occupied by fuel processing components.